Find the principal value of each of the following :
$\cot ^{1}(-1)$
$\cot ^{-1}(-1)=\pi-\cot ^{-1}(1)\left[\right.$ Formula: $\left.\cot ^{-1}(-x)=\pi-\cot ^{-1}(x)\right]$
$=\pi-\frac{\pi}{4}$
$=\frac{3 \pi}{4}$
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